Industrial heterogeneous catalytic systems are normally fabricated by the wet impregnation method. In the wet impregnation method, metal salts are dissolved in solvents and deposited on solid supports by evaporation of the solvents A calcination at high temperature (.gtoreq.500.degree. C.) in air converts the metal salts to metal oxides dispersed on the surface of the solid supports. For example, solutions of ammonium molybdate and nickel nitrate (or cobalt nitrate) in appropriate concentrations deposited on .gamma.-alumina give widely used nickel oxide (cobalt oxide)-molybdenum oxide/aluminum oxide industrial catalysts for hydro-refining heavy oil.
As it is known, by those skilled in the art, that the efficacy, efficiency and selectivity of catalysts depends on the composition, dispersion and surface area of the catalysts. These factors are of paramount importance to the activity and selectivity of the catalysts. The wet impregnation method provides an excellent methodology for metal catalysis, especially for zerovalent metals, but it does not provide the same quality of fabrication for metal oxide catalysts. The additional size and higher valency of metal oxides causes them to clump and aggregate, thereby interfering with the fine, homogeneous, dispersion of nuclear units required for optimal catalytic activity.
An alternative methodology for preparing both liquid phase and solid phase catalysts is by metal vapour synthesis (MVS). The catalytic reagents are vaporized using an electron beam gun or resistive heating. The technology for vaporizing metal atoms is well developed and even refractory metals, such as tungsten (mp 3410.degree. C. at 1 atm), can be vaporized using an electron gun. At the present time, commercially available apparatus and equipment for the vaporization of metal atoms is available in the marketplace. Two examples of commercially available equipment are the device manufactured by G.V. Planar Ltd., Sunbury-on-Thames, in the United Kingdom and the Torrovap apparatus manufactured by Torrovap Industries, Inc., in Canada. The use of metal vaporization for the preparation of zerovalent metals for deposition on solid supports has been shown by Dr. Geoffrey A. Ozin. Dr. Ozin is the inventor or co-inventor of the following patents: U.S. Pat. No. 4,552,855 for zeolite-metal catalysts; U.S. Pat. No. 4,569,924 for carbon-metal catalysts; U.S. Pat. No. 4,292,253 for organo-metal catalysts.
Attempts to use metal vapour synthesis on metal oxides to fabricate catalysts have not been previously described. One reason may be that many metal oxides are highly ionic in nature and tend to decompose upon attempted vaporization. However, some metal oxides such as molybdenum trioxide are known to be volatile without decomposition. It is shown in one preferred embodiment of the present invention that molybdenum trioxide, under appropriate reaction conditions, can be vaporized, cocondensed and precipitated using MVS technology to produce a stable heterogeneous catalyst. To the extent that other metal oxides would produce a mixture of metal atoms and reduced metal oxides the methodology of the present invention is still useful as these can be oxidized again once they have been precipitated. Quite surprisingly the MVS methodology is very forgiving with respect to its use on metal oxides. The ability to solvate the metal oxide into a solvent appears to be the key to this surprising success.